A device comprising a hole injection transport layer is expected to be widely applied to organic devices such as an organic electroluminescence device (hereinafter referred- to as organic EL device), an organic transistor, an organic solar cell and an organic semiconductor, and a broad range of basic devices and uses such as a quantum-dot light emitting device and an oxide-based compound solar cell.
For example, the organic EL device is a charge injection type light emitting device which uses emission of light that occurs when an electron and a hole reach a light emitting layer and they are recombined. Now, various kinds of multilayer structures are proposed to obtain high light emitting efficiency and long life, such as a five-layer structure comprising an electron injection layer/electron transport layer/light emitting layer/hole transport layer/hole injection layer.
The layers other than the light emitting layer, such as the electron injection layer, electron transport layer, hole transport layer and hole injection layer, are said to be effective in making it easy to inject or transport an electrical charge into a light emitting layer, blocking an electrical charge and thus maintaining the balance between electronic current and hole current, or preventing diffusion of light energy excitons.
It has been attempted to increase electrical conductivity by mixing an oxidizing compound with a hole transporting material in order to increase charge transport performance and charge injection performance.
In patent literatures 1 to 4, a metal oxide, which is a compound semiconductor, is used as an oxidizing compound, that is, an electron-accepting compound. For example, a thin film is formed from a metal oxide such as vanadium pentoxide or molybdenum trioxide by a vapor-deposition method, or a mixed film is formed by co-deposition of a molybdenum oxide and an amine-based low molecular compound.
In non-patent literature 1, as an attempt to form a coating film of vanadium pentoxide, a method for forming a charge transfer complex is disclosed, in which a solution in which oxovanadium(V)tri-i-propoxideoxide is dissolved as an oxidizing compound (that is, electron-accepting compound) is used, and after a coating film is formed from the mixture of the solution and a hole transporting polymer, the coating film is hydrolyzed with water vapor to make a vanadium oxide, thereby forming a charge transfer complex. In non-patent literature 1, however, the solution is solidified by hydrolysis and polycondensation reaction, so that the vanadium is likely to aggregate and film quality control is difficult. Therefore, no excellent film is obtained. The reason why oxovanadium(V)tri-i-propoxideoxide is mixed with a hole transporting polymer is that no coating film can be formed with oxovanadium(V)tri-i-propoxideoxide only, so that the coating film of non-patent literature 1 inevitably has a high organic component concentration and a poor vanadium concentration, which is considered to be a component that is effective in increasing the life of devices. Accordingly, further improvement in life and device properties is required for non-patent literature 1.
In non-patent literature 5, an attempt to form a coating film of molybdenum trioxide is described, in which fine particles produced by physically pulverizing molybdenum trioxide are dispersed in a solution to form a slurry, and a hole injection layer is formed by applying the slurry.
However, even though the oxidizing materials as disclosed in patent literatures 1 to 5 and non-patent literature 1 are used for hole transporting materials, it is difficult to obtain a long life or a further increase in life is needed. The metal oxides disclosed in patent literatures 1 to 4 can increase hole injection performance to some extent; however, interface adhesion to an adjacent organic compound layer becomes poor, and it is considered to have a negative effect on the life property.
In patent literature 5, it is described that a charge injection layer was produced by using a slurry obtained by dispersing oxidized molybdenum particles having an average particle diameter of 20 nm in a solvent and by a screen printing method. However, in the case of using the method of pulverizing MoO3 powder as disclosed in patent literature 5, it is actually very difficult to produce fine particles of uniform size having a diameter of 10 nm or less to meet the request for forming a hole injection layer having a thickness of about 10 nm, for example. Also, it is very difficult to disperse oxidized molybdenum particles produced by pulverization stably in a solution, without aggregating the particles. If the solution of fine particles is unstable, only a film with large convexoconcaves and poor flatness can be formed from the solution, which leads to short circuit in devices. If a thin film can be formed only by the vapor-deposition method, there is a problem that even if a light emitting layer is formed by separately applying materials by a solution coating method such as an ink jet printing method, the advantages of the solution coating method are not fully utilized. That is, in order not to damage the liquid repellency of a bank between light emitting layers by a molybdenum oxide to be lyophilic, it is needed to deposit a hole injection layer or hole transport layer containing a molybdenum oxide, which is an inorganic compound, using a high resolution mask. Therefore, in terms of cost and yield, the advantages of the solution coating method cannot be utilized. In addition, the molybdenum oxide, which is an inorganic compound, is an oxygen-deficient oxide semiconductor, so that in terms of electrical conductivity, Mo2O5 having an oxidation number of +5 is a better conductor at normal temperature than MoO3 having an oxidation number of +6. However, Mo2O5 having an oxidation number of +5 is unstable in the air; therefore, compounds that can be thermally vapor-deposited thereon easily are limited to oxidized compounds having a stable valence such as MoO3 and MoO2.
A film forming property and the stability of a thin film are largely concerned with the life property of devices. In general, the life of organic EL devices is a time to half-luminescence when the device is continuously driven in a given current operation, and it is said a device with a longer time to half-luminescence has a longer life.
In display production using an EL device, etc., a light emitting layer and so on are generally formed by patterning. Various kinds of patterning methods are proposed as the method for forming a light emitting layer and so on, such as a method for vapor-depositing materials through a shadow mask, a method for separately applying materials by ink jet printing, a method for transferring emitting dyes, a flexographic printing method and a gravure printing method. In the method for separately applying materials by ink jet printing, to order to form a high-resolution, fine pattern, it is proposed to form a bank and perform an ink repellent treatment on the bank surface by a plasma treatment with fluorine gas or the like (for example, see patent literature 6) and it is also proposed to form a bank using liquid repellent materials (for example, see patent literature 7). In addition, as the method for forming a light emitting layer by patterning, a method using a photocatalyst (for example, patent literature 8) and a method using vacuum ultraviolet light are proposed, both of which methods are able to form a high-resolution pattern.
In the above method of performing an ink repellent treatment on the bank surface by a plasma treatment with fluorine gas or the like, however, the heat resistance of the ink repellent treatment is low, so that problems are caused in subsequent processes. For example, there is a problem that due to heating at relatively high temperature (such as 200° C.) which is performed when a layer is formed in an opening of the bank after the ink repellent treatment, fluorine introduced into the bank is eliminated to provide the bank with ink-affinity; therefore, layers cannot be stacked and the properties of the resulting device are deteriorated. Also in the case where the bank is formed with an ink repellent material, in many ink repellent materials, fluorine is likely to be eliminated therefrom by heating at relatively high temperature and such materials have no resistance; therefore, they have a similar problem to the above. By such a method, a bank in which the top is ink repellent but the side has ink affinity, cannot be formed.
The above method which uses a photocatalyst and that which uses vacuum ultraviolet light utilize such a phenomenon that, in a part affected by photocatalytic action associated with energy irradiation or in a part irradiated with vacuum ultraviolet light, the wettability of the part is altered so as to lower the liquid contact angle thereof. In particular, the methods utilizes the phenomenon that the part affected by photocatalytic action associated with energy irradiation or the part irradiated with vacuum ultraviolet light becomes lyophilic region, while a part not affected by photocatalytic action associated with energy irradiation or a part not irradiated with vacuum ultraviolet light becomes a liquid repellent region. Because of this, a light emitting layer or the like is formed on the part affected by photocatalytic action associated with energy irradiation or the part irradiated with vacuum ultraviolet light. However, when a layer having alterable wettability has a hole transporting property or the like, there is a material deterioration in the part affected by photocatalytic action associated with energy irradiation or the part irradiated with vacuum ultraviolet light, resulting in a problem of damage to the hole transporting property or the like.